At present, environmental regulations become more and more severe on global scale. Specifically, circumstances surrounding the automobile industry, such as fuel consumption regulations and exhaust gas emission regulations, become increasingly more severe. In the background of such severe regulations, there are environmental problems, such as global warming, and necessity for resource conservation as a result of possible depletion of petroleum resources. Therefore, reduction of fuel consumption of automobiles is considered to be further advanced.
In order to reduce fuel consumption in automobiles, not only an improvement of automobiles per se, such as reduction of the weight thereof and improvement of engines but also an improvement of an engine oil, such as reduction of the viscosity thereof and addition of a good friction regulating agent for the prevention of frictional loss in the engine, is also important. A reduction of the viscosity of an engine oil, however, causes an increase of wear of engine components. Thus, for the purpose of reducing frictional loss, which might be caused by a reduction of the viscosity of an engine oil, and preventing wear, a friction modifier, an extreme pressure agent, etc. are added. As the extreme pressure agent, generally used is a phosphorus-containing compound. The phosphorus-containing compound is, however, known to deteriorate a catalyst used for purifying the exhaust gas. Therefore, it is desired to reduce the amount of such a phosphorus-containing compound in the engine oil as much as possible.
In diesel engines, it is an important problem to provide a method for reducing environmental pollution caused by exhaust gas components such as particulate matter (PM) and NOx. One effective solution to this problem is to mount an exhaust gas purifying device using a particulate filter or an exhaust gas purifying catalyst (oxidation or reduction catalyst) on automobiles. When a conventional lubricating oil for internal combustion engines is used in an automobile provided with such an exhaust gas purifying device, a problem arises because, although soot which deposits on the particulate filter is removed by oxidation and combustion, the filter is clogged by metal oxides, phosphate salts, sulfate salts and carboxylate salts that are formed in situ by the combustion. Part of the engine oil used is combusted and discharged as an exhaust gas. Therefore, it is preferable to maintain the content of metal components and sulfur components in the lubricating oil at as low level as possible. It is also preferable to reduce the content of phosphorus components and sulfur components in the lubricant oil for reasons of prevention of catalyst degradation.
In this circumstance, a lubricating oil composition for a diesel engine provided with a diesel particulate filter (DPF) is disclosed which is capable of reducing clogging of DPF by an ash component, capable of improving the combustibility of PM caught by DPF, capable of stably combusting PM at a low temperature, capable of improving the removing efficiency thereof, and capable of prolonging the service life of DPF (see, for example, Patent Document 1). The lubricant oil composition for a diesel oil engine provided with a device for removing diesel particulate matters is characterized in that the composition has a sulfated ash content of 1.0% by weight or less, a sulfur component content of 0.3% by weight or less and a molybdenum content of 100 ppm or more.
As an extreme pressure additive for a lubricant oil, a phosphorus-based or sulfur-based extreme pressure agent has been hitherto generally used. The extreme pressure agent which contains a phosphorus atom and/or a sulfur atom in its molecule is dissolved or uniformly dispersed in a base oil to exhibit the extreme pressure behavior. Examples of the known extreme pressure agent include metal salts of dithiophosphoric acid, sulfurized fats, sulfurized fatty acids, sulfurized esters, polysulfides, sulfurized olefins, thiocarbamates, thioterpenes and dialkyl thiodipropionates. These extreme pressure additives, however, have problems that metals are corroded by them, seizure preventing effect is not sufficiently achieved due to interaction thereof with other additives and oxidation stability is insufficient and, therefore, are not fully satisfactory.
In this circumstance, the present applicant has developed a novel sulfur-based extreme pressure additive which has better load carrying capacity and wear resistance as compared with the conventional extreme pressure additive, which is less corrosive to metals, which can be used for lubricating oils and which comprises a disulfide compound having a specific structure (see, for example, Patent Documents 2 and 3).
It is known that an improvement of wear resistance, an improvement of oxidation stability, an improvement of anti-deposit property and an improvement of reduction of friction may be obtained, when a molybdenum-containing additive is added to a lubricating oil such as an engine oil for passenger cars, a heavy-load diesel engine oil or a natural gas engine oil (see, for example, Patent Documents 4 to 10). As the molybdenum-containing additive, there may be mentioned, for example, organomolybdenum compounds such as sulfurized oxymolybdenum dithiocarbamate, sulfurized oxymolybdenum dithiophosphate, sulfurized oxymolybdenum dithioxanthogenate, molybdenum-amine complexes, trinuclear molybdenum-sulfur compounds and sulfur-containing molybdenum complexes of succinimide.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-60776    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-262964    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2006-45335    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2001-262175    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2003-252887    [Patent Document 6] Japanese Translation of PCT International Application Publication No. 2003-523454    [Patent Document 7] Japanese Translation of PCT International Application Publication No. 2003-500521    [Patent Document 8] Japanese Unexamined Patent Application Publication No. 2004-51985    [Patent Document 9] Japanese Examined Patent Publication No. H03-22438    [Patent Document 10] Japanese Unexamined Patent Application Publication No. 2004-2866